Temperature responsive by-pass and like control valves frequently comprise a housing formed with fluid flow apertures and containing one or more valve elements settable to control fluid flow through the housing. One or more power type thermal actuators are in the housing, and, responsive to changing fluid temperature, effect positive movements of the valve element or elements to different control positions. Power type thermal actuators are known in the art and are known to have a high degree of reliability. Nevertheless, they do sometimes fail in use. Failure may be evidenced by an inability properly to project a movable part, or may be evidenced by a complete inability to operate. In either event, corresponding failure of the valve to perform its intended purpose results.
In instances where the probability of valve failure must be reduced to a minimum, it has been proposed to construct the valve with duplicate or redundant thermal actuators. The plural actuators would operate in unison with one another on the same valve element or elements. Should one thereof fail, another would remain in service and be capable of effecting all necessary valve operations. Problems exist in incorporating redundant units in a valve structure so that they are equally responsive to fluid temperature change. Also, should one of the thermal actuators fail, there is nothing to give warning that failure has occurred since the remaining thermal actuator continues a normal valve operation. At any given time in the operation of a fluid flowing system, therefore, there can be no assurance that the protection afforded by redundant thermal actuators does in fact prevail. Nor in a pre-use check out of a system can it be determined that a redundant thermal actuator is operating improperly or not at all.
Insofar as is known, the prior art contains no teaching of methods or apparatus providing for effective use of redundant thermal actuators, and no means to carry out safety inspections in connection with either single or redundant actuators.